In recent years, vigorous moves toward thin and compact design have been under way in the field of small information apparatuses such as HDDs (Hard Disk Drives), mobile computers, IC cards, and the like, as well as in the field of mobile communication apparatuses such as portable telephones, car telephones, paging systems, and the like. With this trend, the need for reducing the size and thickness of crystal devices, exemplified by crystal oscillators, used in such apparatuses has been increasing.
Among such crystal devices, particularly, in the case of gyro sensor devices used for angular velocity detection in navigation systems or for camera shake control in video cameras, not only the need for a thin and compact design but the need for high accuracy has also been increasing.
With this trend toward thin and compact design and high accuracy, it has become important that a tiny crystal plate, sliced from a crystal substrate, be mounted in a package accurately and under a constant and suitable pressure.
A 32.768-kHz crystal oscillator for a watche, which uses a tuning fork crystal plate, is one known example of such a crystal device (for example, patent document 1: JP-A-2002-9577, page 4 and FIG. 18).
FIG. 33 is a cross-sectional view of an essential portion schematically showing the structure of the above crystal oscillator.
The crystal oscillator 80 has a structure in which the crystal plate 81 sliced in the shape of a tuning fork from a crystal substrate, and provided with metal electrodes (not shown) on its major surfaces for driving purposes, is bonded by an adhesive 84 to a mounting base 83 provided inside a package 85 formed from a ceramic material and is sealed in a vacuum atmosphere by closing the structure with a lid member 86 formed from a transparent glass or ceramic material.
The crystal oscillator 80 is fabricated in the following manner.
First, as shown in FIG. 33, a tiny through-hole communicating between the interior and the exterior of the structure is formed in advance through the package 85.
Then, the base portion of the crystal plate 81 is bonded using the heat-hardenable adhesive 84 to the mounting base 83 provided inside the package 85.
Next, a first sealing step is performed in which the lid member 86 is placed on the package 85 and they are joined together.
Next, a second sealing step is performed. In the second sealing step, a metal sealing material 87 is inserted in the through-hole of the package 85 in a vacuum atmosphere, and a laser beam or an electron beam is applied to the sealing material 87 to heat it using the energy of the laser beam or electron beam. This causes the sealing material 87 to melt and close the through-hole, thus sealing the interior of the package 85 in a vacuum condition.
Other crystal devices, such as crystal oscillators and gyro sensor devices, are also fabricated in substantially the same manner as that described above.
In the fabrication of the above crystal oscillator, the step of bonding the crystal plate 81 to the mounting base 83 provided inside the package 85 has been performed by bonding the crystal plate 81 to the base 83 by the adhesive 84 after suitably positioning the crystal plate 81 on the mounting base 83 which is made somewhat larger than the base portion of the crystal plate 81.
FIG. 34 is a diagram showing examples of bonding failures that can occur in the thus fabricated crystal oscillator.
When the adhesive 84 is hardened by heating, the adhesive 84 undergoes changes in viscosity and changes in stress. At this time, due to the effects of the surface conditions (such as wettability, surface roughness, and contamination) of the mounting base 83, the crystal plate 81 may be pulled unexpectedly in an unintended direction, which can often result in an situation such as shown in FIG. 34A, where the crystal plate 81 is bonded by being displaced from the center axis of the package 85, or in a situation such as shown in FIG. 34B, where the crystal plate 81 is bonded with its vibrating prongs 82 inclined at an angle θ with respect to the center axis of the package 85.
If the crystal plate 81 is bonded obliquely inside the package 85, in the worst case the crystal plate 81 may come into contact with the package 85. If this happens, vibrations may not be produced as designed, or in some cases, the vibrating prongs 82 may be broken. Since such defects degrade reliability, the prior art crystal oscillator has been designed by making the package somewhat larger in size in order to avoid such defects. As a result, in the prior art, it has been difficult to reduce the size of the package. This problem is not limited to crystal oscillators, but can occur in crystal devices in general.
One application of the crystal device is the gyro sensor device which is used in a navigation system for detecting the position of a vehicle, etc. In the gyro sensor device, the mounting angle of the crystal plate inside the package greatly affects the accuracy of detection of the angular velocity. Usually, in the gyro sensor device, the crystal plate is mounted with its vibrating prongs oriented parallel to the spinning axis Z of the gyro sensor device, and the vibrating prongs are caused to vibrate in directions perpendicular to the spinning axis Z, thereby detecting an accurate angular velocity Ω. However, with the crystal plate bonding method as used in the prior art, it has been difficult to accurately orient the crystal plate with respect to the package, resulting in the problem that a gyro sensor device often has poor accuracy.
It is also known to provide a surface-mount type piezoelectric device that can be mounted directly on the surface of the circuit board of an apparatus (for example, JP-A-2003-152499, page 5 and FIGS. 3 and 4).
FIG. 35 is a cross-sectional view schematically showing the structure of the above piezoelectric device.
The piezoelectric device 90 contains a piezoelectric oscillator 92 inside a package 91. The package 91 is a substrate made, for example, from a sintered aluminum oxide structure formed by sintering a stack of ceramic green sheets, and is formed in the shape of a shallow box. A prescribed interior space S is formed inside the package of the stacked structure. On the bottom of the interior space S, Au- and Ni-plated electrodes 93, spaced a prescribed distance apart from each other, are formed near the edge portion in the width direction of the package 91. The electrodes 93 are connected to an external circuit for supply of a driving voltage.
FIG. 36 shows an enlarged view of the portion where the piezoelectric oscillator 92 is bonded to the electrodes 93 in the piezoelectric device 90.
A silicone-based conductive adhesive 94 is applied on each electrode 93. The base portion of the piezoelectric oscillator 92 is placed on the conductive adhesive 94 and pressed lightly, causing the conductive adhesive 94 to spread. When the conductive adhesive 94 is hardened, the piezoelectric oscillator 92 is bonded to the electrodes 93. When the conductive adhesive 94 is applied and pressed lightly, the spreading conductive adhesive 94 is blocked by a groove 96 formed around each lead electrode 95 of the piezoelectric oscillator 92. Accordingly, the electrodes 93 do not contact each other, thus effectively preventing a short circuit. The open top of the package 91 is closed by bonding a lid member 97 using a brazing flux such as low-melting-point glass. The lid member 97 is formed from an optically transmissive material, for example, glass so that the frequency can be adjusted using laser light passing through the lid member 97.
In a gyro sensor device that uses the above piezoelectric device 90, an AC voltage is applied across the electrodes 95 of the piezoelectric oscillator 92, causing it to vibrate in a driving direction at a velocity v and thus producing vibrations at its natural frequency in the driving direction. In this condition, the tuning fork portion of the piezoelectric oscillator 92 spins at an angular velocity co about its center axis extending along the longitudinal direction of the tuning fork portion, and a Coriolis force of F=2 mvω is generated in the piezoelectric oscillator 92. In the gyro sensor device using the above piezoelectric device 90, the angular velocity is detected based on the output voltage generated by the vibrations caused by the Coriolis force. The gyro sensor device is constructed so that the prongs of the tuning fork crystal oscillator are oriented in a prescribed direction relative to the generating direction of the Coriolis force to be detected. Here, if the prongs of the tuning fork crystal oscillator are inclined relative to the generating direction of the Coriolis force, the Coriolis force generated in the prongs is a component of force proportional to the inclination, and the output voltage generated is inaccurate, degrading the detection accuracy. Therefore, in the gyro sensor device, there has been a need to increase the detection accuracy by increasing the mounting accuracy of the tuning fork oscillator.
In the above piezoelectric device 90, when bonding the base portion 92a of the piezoelectric oscillator 92 to the ceramic package 91, there arises the possibility that, due to variations in the amount and position of the applied conductive adhesive 94, the balance of the surface tension of the conductive adhesive 94, between the two electrodes, may be disrupted and the base portion 92a of the piezoelectric oscillator 92 may be bonded obliquely. If the piezoelectric oscillator 92 is not mounted correctly in position, there arises the problem that the performance becomes unstable. Further, in order to increase the mounting accuracy of the piezoelectric oscillator 92, the piezoelectric oscillator 92 must be held in position using a positioning jig or the like until the conductive adhesive 94 hardens. However, the use of such a positioning jig leads to the problem that the work efficiency of the bonding step of the piezoelectric oscillator 92 is degraded.